Count cells that do not contain

COUNTIF counts the number of cells in the range that do not contain "a" by matching the content of each cell against the pattern "<>*a*", which is supplied as the criteria. The "*" symbol (the asterisk) is a wildcard in Excel that means "match any number of characters" and "<>" means "does not equal", so this pattern will count any cell that does not contain "a" in any position. The count of cells that match this pattern is returned as a number.

With a cell reference

You can easily adjust this formula to use the contents of another cell that contains the text you do not want to count. The generic form of the formula looks like this:

=COUNTIF(rng,"<>*"&a1&"*")

Exclude blanks

To also exclude blank cells, you can switch to COUNTIFS and add another criteria like this:

=COUNTIFS(range,"<>*a*",range,"?*") // requires some text